1. Field of the Invention
This invention relates to packages for semiconductor devices, and more particularly, this invention relates to packages for integrated circuits which have leads connected together that are severed through recesses in the package after the formation of the package with the semiconductor device enclosed therein.
2. Description of the Prior Art
Various types of lead frames and the construction of packages for semiconductor devices, such as integrated circuits, are known in the prior art. An example of a package for an integrated circuit is shown in U.S. Pat. No. 3,781,457 by McKorreghan, issued on Dec. 25, 1973 and entitled "Glass Dual-In-Line Header". It is desirable for various reasons to hermetically seal the integrated circuit within the package. Therefore, the package should completely enclose the central area in which the integrated circuit is located. This prevents the movement of air and in particular moisture into the central area where the integrated circuit is located.
One type of lead frame is utilized in conjunction with ceramic packages for integrated circuits and in particular to those types of integrated circuit packages known as ceramic dual in line packages (C-DIP). This type of package provides a complete enclosure for the integrated circuit and allows the integrated circuit to be connected to each of the leads of the lead frame within the enclosure with one end of the lead located adjacent to the central area in which the integrated circuit is located. The other end of the lead extends out of the enclosure for contact with another electrical component, for example, a printed circuit board. It is, therefore, desirable that each of the leads be isolated from all the other leads both within the enclosure of the package and outside the enclosure of the package. The lead frames utilized with certain types of ceramic packages have a so-called flying lead. Each of the leads has an end disposed generally about a central portion of the lead frame which overlays the central area of the package when assembled. The exterior ends of the leads are connected to a common rail. Typically, the exterior portions of the leads are parallel on two opposite sides of the lead frame with two separate rails provided. The flying leads are attached at one end to one end of the rail and at another end to an end of the other opposite rail. The flying leads extend between the rails at an angle of approximately 90.degree. to the rails. After the lead frame is formed into an appropriate shape for utilization within the enclosure of the package, the flying leads are of the same general shape, as the other leads and still interconnect the rails of the lead frame.
The external portions of the frame are formed and bent to approximately an 80.degree. angle with respect to the portion of the lead frame which is located within the enclosure of the package. The bends in the leads are approximately parallel to the rails. The package is then assembled, as is discussed herebelow in conjunction with the detailed description of the present invention. A contact pad on the integrated circuit is connected to a contact pad located adjacent to the end of each of the leads. The package is then hermetically sealed with the integrated circuit connected to each of the leads within a central area. The rails are then severed from the ends of the leads outside the package, which as a result, severs the flying leads from the assembled package. Only the leads which connect to the integrated circuit within the package remain.
Thus, the circuit from a contact pad on the integrated circuit through a wire loop extending from that contact pad to the associated contact pad of the lead frame and the connection through the enclosure of the package to the end of the lead outside the package is isolated from all the other leads. This provides a complete electrical circuit for connecting the contact pads of the integrated circuit to another electrical component, for example, a printed circuit board. It has, after formation of the hermetically sealed device, been found useful to plate the exterior portions of the leads with tin. During plating, it has been found useful to have the flying rails attached between the end rails of the lead frame. Thus, the plating step takes place between the hermetic sealing of the integrated circuit within the enclosure of the package and the severing of the rails from the leads. The flying rails constitute approximately 25% of the metal and weight on the lead frame and also greatly widen the lead frame. If the flying rails can be eliminated, more lead frames can be formed from the same amount of metal with the weight of each individual lead frame being reduced. Further, after the packages are formed, the flying rails can, in automatic devices, overlap and lock with the flying rails on other adjacent packages which can jam and reduce the efficiency of automated handling equipment.
None of the prior construction shows a package having recesses located in the enclosing structure about the integrated circuit which allows the severing of connected leads after hermetically sealing the chip carrier.